1. Field of the Invention
The invention relates to a method for performing plasma processing on a substrate and, more particularly, to a method for processing a substrate with a low pressure plasma and a high pressure plasma in the same plasma processing system.
2. Description of Related Art
During semiconductor processing, plasma is often utilized to assist etch processes by facilitating the anisotropic removal of material along fine lines or within vias (or contacts) patterned on a semiconductor substrate. Furthermore, plasma is utilized to enhance the deposition of thin films by providing improved mobility of adatoms on a semiconductor substrate.
For example, during dry plasma etching, a semiconductor substrate having an overlying patterned, protective layer, such as a photoresist layer, is positioned on a substrate holder in a plasma processing system. Once the substrate is positioned within the chamber, an ionizable, dissociative gas mixture is introduced, whereby the chemical composition is specially chosen for the specific material being etched on the semiconductor substrate. As the gas is introduced, excess gases are evacuated from the plasma processing system using a vacuum pump.
Thereafter, plasma is formed when a fraction of the gas species present are ionized by electrons heated via the transfer of radio frequency (RF) power either inductively or capacitively, or microwave power using, for example, electron cyclotron resonance (ECR). Moreover, the heated electrons serve to dissociate some species of the ambient gas species and create reactant specie(s) suitable for the exposed surface etch chemistry. Once the plasma is formed, selected surfaces of the substrate are etched by the plasma.
The process is adjusted to achieve appropriate conditions, including an appropriate concentration of desirable reactant and ion populations to etch various features (e.g., trenches, vias, contacts, etc.) in the selected regions of the substrate. Such substrate materials where etching is required include silicon dioxide (SiO2), low-k dielectric materials, poly-silicon, and silicon nitride.
The use of plasma (i.e., electrically charged particles), for example during etching, facilitates the anisotropic removal of material on the substrate in high aspect ratio features. Due to the charge of an ion or electron in the plasma, these particles may be effectively manipulated and guided using an electric field. In some applications where a high degree of anisotropy is required (e.g., polycrystalline silicon etching in front-end-of-line (FEOL) structures), low pressure processing may be required to limit the number of collisions between the directional flow of ions and the background gas. For instance, in an argon plasma, the ion-neutral mean free path is about 3 cm (centimeters) for a pressure of 1 mtorr (millitorr) and it is about 0.15 cm for a pressure of 20 mtorr. Therefore, low pressure processes (e.g., less than about 20 mtorr) may be more suitable for increased directionality for ions incident on the substrate and, hence, etch anisotropy. However, plasma formation and heating are more difficult for such low pressure processes.